1. Field of the Invention
The present invention relates to an image forming apparatus such as electrophotographic copiers, printers and the like.
2. Description of the Related Art
In image forming apparatus such as electrophotographic copiers and printers and the like, generally, a photosensitive member is charged by a charging device, and the charged region is subjected to optical image exposure so as to form an electrostatic latent image thereon. This latent image is developed into a visible toner image which is transferred onto a transfer member and fixed thereon. After the aforesaid transfer, the residual toner remaining on the surface of the photosensitive member is removed therefrom by a cleaning device.
In recent years, various apparatus have been proposed which omit a cleaning device in accordance with demand for lower cost and more compact apparatus.
For example, U.S. Pat. No. 5,148,219, U.S. Pat. No. 5,283,618, and U.S. Pat. No. 5,328,792 disclose so-called cleanerless image forming apparatus which combine a cleaning device with a developing device.
In cleanerless image forming apparatus, however, inadequate charging and subsequent inadequate optical exposure (so-called exposure memory) readily occurs when a charging process and optical exposure process are accomplished over residual toner remaining on the surface of the photosensitive member following a previous toner image transfer. As a result, a so-called residual toner memory and inadequate exposure memory are developed in a subsequent image, thereby producing a defective image.
In order to eliminate the aforesaid problems, U.S. Pat. No. 5,148,219 discloses an agitating/charging device capable of disturbing the untransferred residual toner into a non-pattern which is used as a charging device; and U.S. Pat. No. 5,283,618 discloses settings of developing toner resistance value, developing toner charge amount, and residual toner charge amount within predetermined ranges.
In image forming apparatus having cleanerless construction, it is difficult to both maintain suitable density of the formed image and collect the post-transfer residual toner by a developing device. Thus, U.S. Pat. No. 5,328,792 discloses an improvement of both image density and cleaning characteristics by using as a developer a novel nonmagnetic monocomponent developer.
So-called cleanerless image forming apparatus still have the disadvantages described below, however.
In image forming apparatus having a cleaning device, residual toner remaining on the surface of a photosensitive member is removed by a cleaning member such as a blade or a brush and the surface of the photosensitive member is shaved at the same time. When a cleaning device is not used, however, a toner film readily forms on the surface of the photosensitive member because there is no member present to suitably shave the photosensitive member. This toner filming is a phenomenon that a thin film of toner adheres over the entire surface of the photosensitive member.
If conventional toner is used, this toner filming does not become a visible problem insofar as so much repeated image formations are not performed. However, when low melting point toner i.e., toner having Ti (the softening point at the moment the plunger of the flow tester starts to drop) less than 120.degree. C. and Tm (the softening point at the moment the plunger of the flow tester has drop 6 mm) less than 135.degree. C. is used for power conservation, and the fixing temperature of the toner image on the transfer member is reduced, this toner filming easily occurs. When this low melting point toner is used, toner filming occurs after several thousand image forming processes, and is believed to be caused by the softening of the low melting point toner which causes it to readily fuse to the photosensitive member.
When toner filming occurs, the charge flows horizontally on the surface of the photosensitive member when an electrostatic latent image is formed particularly under environmental conditions of high temperature and high humidity, thereby causing so-called image drift. For example, when image drift occurs in the case of reversal development wherein toner adheres to the exposure region, in the formation of the halftone dot image shown in FIG. 2, the location d1 at which the surface potential of the photosensitive member (e.g., -600 V) is reduced at a dot (e.g., to -50 V), as shown in FIG. 3A receives a charge flow as shown in FIG. 3B, such that the dot to be developed in a black color or the like is eliminated.